Change the line #4 of your code for this one:
<button style="background-color:#000000; color:white; padding-top:8px; padding-bottom:8px; padding-left:30px; padding-right:30px;"id="eventbrite-widget-modal-trigger-55166464380" type="button">Buy Tickets</button>
You can change the color of the background by changing the #000000 value. The “color:white” is for the font. You can also change the size of the button by augmenting all the padding values (so the entire button is clickable unlike your actual version where only the text is clickable)
IMPORTANT: you will have to delete the width, height and background color values of your embed element.
Hope this helps! 